The present invention relates to a subscriber line interface circuit for telephone exchanges.
A subscriber line interface circuit as shown and described in articles "A Versatile Subscriber Line Interface Circuit" by Nishimura et al, ESSCIRC-83 (European Solid-State Circuit Conference 1983) and "General-Purpose Subscriber Circuit Using High-Voltage LSI for NEAX-61 Fully Digital Local Switch" by Arai et al, ICC-84 (International Conference on Communication 1984), employs a high-impedance feedback circuit and a plural feedback circuits which synthesizes desired impedances according to different frequency bands. This subscriber line circuit has a wide range of adaptability to various operating conditions which may be encountered in telephone exchanges.
Another prior art circuit is disclosed in European Patent Application No. 0,147,230. This prior art provides high precision impedance synthesis by a single-amplifier high-impedance feedback configuration.
However, in these prior art interface circuits DC and AC feedback circuits are coupled to the positive and negative input terminals of a balanced amplifier. As a result of this configuration, these prior art circuits are still unsatisfactory from the circuit complexity and power consumption points of view if they are implemented by LSI (Large Scale Integration) technology.
In addition, while the prior art described in the aforesaid articles is capable of setting a feed resistance at a desired value of 200 ohms.times.2 or 400 ohms.times.2 with the use of two resistors connected in the tip and ring lines to the outputs of a balanced amplifier and capable of setting a constant current at a desired value of 35 mA or 80 mA with a current limiter in a DC feedback circuit, the variation of the feed resistance accompanies an undesirable variation of the constant current and a further undesirable variation of a threshold by which the subscriber loop is monitored for detection of on- and off-hook conditions.
Furthermore, it is important for telephone services to ensure that subscriber line interface circuits have good longitudinal balance even when the tip-ring voltage of the interface circuit goes beyond the active bias region of the LSI circuitry when the subscriber line goes on-hook. The aforesaid prior art circuits have a constant DC feedback control characteristic by which a predetermined amount of DC current is fed back to an output circuit to cause the tip-ring voltage to be controlled in the active bias region which is typically a few volts within the range between ground and battery voltage during on-hook conditions. However, in a subscriber line interface circuit in which the feed resistance is variable, the constant current DC feedback would result in a variation of the bias voltage due to interaction between the variable feed resistance and the constant DC feedback current.